The overall objective is to further define the site and mechanism of the positive inotropic and the arrhythmic (toxic) effect of digitalis, to determine whether digitalis treatment results in a change in the rate of synthesis of Na-K-ATPase in the heart, and to continue to study the properties of new digitaloids and the interaction of digitaloids with membranes. BIBLIOGRAPHIC REFERENCES: The Uptake and Subcellular Distribution of Radio-Labelled Metabolites of Digitoxin in the Guinea-Pig Isolated Perfused Heart. S. Dutta, B.H. Marks and P.M. Stephen. Br. J. Pharmac. (1976), 56, pp. 437-441. Cardiac NaKATPase During Positive Inotropic and Toxic Actions of Ouabain. H.M. Rhee, S. Dutta and B.H. Marks, Eur. J. Pharmacol. 37, pp. 141-153 (1976).